Anisimkin discovered numerically and verified experimentally a quasilongitudinal mode (below describe as “QL mode”), which is a special propagation mode to propagate along X-axis direction in a quartz ST cut (non-patent document 1).
Considering a guided wave propagated in a plate that consists of an anisotropy crystalline material and assuming that a longitudinal wave component is a P wave component, a transverse wave component having a displacement vertical to a surface of the plate is an SV wave component and a transverse wave component having a displacement parallel to the above-mentioned surface is an SH wave component, the above-mentioned QL mode has the features, such as (i) there is only the P wave component on the surface of the plate, (ii) inside the plate as well, the P wave component is larger than the SV wave and the SH wave components. Specially, the feature of (i) is suitable for applying to a piezoelectric sensor, which uses a piezoelectric crystalline material for sensing an adsorption of a to-be-sensed object to a piezoelectric plate surface or detecting a viscosity change of a fluid contacting with the piezoelectric plate surface.
The inventors analyzed conditions that the QL mode and a quasishear mode (below described as “QS mode”) are excited for quartz plates, wherein the QS mode is which the P wave component becomes zero and the SH component and the SV component remain on the surface of the plate (non-patent document 2). For further improvement in sensitivity and energy efficiency of the piezoelectric sensor using these QL and QS modes, a piezoelectric crystalline material with high oscillatory frequency in the same plate thickness and high electromechanical coupling factor is required.
In the patent document 1, a surface acoustic wave device to excite a quasilongitudinal type leaky surface acoustic wave on the quartz plate by using an IDT electrode is disclosed. However, the QL mode and the QS mode are bulk waves (lamb waves), in which the P wave component, etc., is excited inside the plate as well; therefore, it is different from the technique in the patent document 1 which uses the surface acoustic wave. In addition, in the patent document 1, there is no description about any piezoelectric sensor for detecting a viscosity of a to-be-sensed object or a fluid.